1. Field of the Invention
The present invention relates to a solid imaging device and a method of manufacturing the same, and particularly relates to a frame transfer or a full frame transfer-type solid imaging device, which is provided with the improved sensitivity and resolution without reducing the transfer efficiency or the transfer charge.
2. Background Art
When CCD type solid imaging devices are classified by their operation mode, there are two systems: one is an interline transfer system and another one is a frame transfer system (or a full-frame transfer system). In the solid imaging device operated by the interline transfer system, each pixel is constructed by a PN junction, and light is incident on the N-type region through the insulating film formed on the region. A vertical CCD resistor is formed adjacent to each pixel in sequence, and the signal charge accumulated on the light receiving portion is transferred to the vertical CCD resistor. The content of the vertical resistor is transferred and output to the horizontal CCD resistor. In contrast, in the frame transfer-type solid imaging device, the CCD is divided into the light receiving portion and the charge storing portion, the signal charge accumulated in the light receiving portion is transferred to the charge storing portion, and the signal charge stored in the charge storing portion is output to the horizontal CCD resistor. In the case of the frame transfer, the transfer of signals is carried out during unoccupied time, and the light receiving portion stores the next signal charge, during the reading period. Therefore, in some cases, as the light receiving portion in the frame transfer type solid imaging device, the other type of CCD is used in which light is admitted through the transparent electrode forming a transfer gate and the photoelectric conversion is conducted at the PN junction below the transfer gate.
The solid imaging device of the full-frame transfer system comprises a light receiving cell array and a horizontal transfer portion, and when the accumulated charge is transferred to the horizontal transfer portion it is necessary to intercept incident light by means of a shutter such as a mechanical shutter. Since the frame transfer type solid imaging device comprises the light receiving cell array portion, the storing portion and a horizontal transfer portion, the charge accumulated in the storing portion is collectively transferred to the storing portion at high speed. Furthermore, since the storing portion is shaded, transfer of the image information stored in the storing portion can be completed by the time the next image information is accumulated in the light receiving portion, it is not necessary to provide a shutter, such as a mechanical shutter, for intercepting the incident light.
FIG. 7 is a cross-sectional diagram showing a structural example of conventional frame transfer type solid imaging devices. The diagram shown in FIG. 7 illustrates the cross-section of the solid imaging device along the longitudinal direction of the transparent electrode. This solid imaging device comprises, on the P-type silicon substrate, an N-type region 52 corresponding to a photoelectric conversion region 51, and a P+-type region 54 corresponding to a channel stop region 53, which separates the adjacent photoelectric regions from each other. Furthermore, a transparent film 56 is formed through an insulating layer 55 on the substrate 50, and a flattening layer 57 is formed on the transparent film 56.
A manufacturing process of the solid imaging device having the above construction will be described. First, as shown in FIG. 8(a), the N-type region 52 and the P+-type region 54 are formed on the P-type silicon substrate 50, the insulating layer 55 and a polycrystalline silicon film are formed in sequence, and an elongated transparent electrode 56 is then formed by patterning the polycrystalline silicon. Subsequently, as shown in FIG. 8(b), a flattening layer 57 made of silicon oxide is formed so as to cover the transparent electrode 56.
In this solid imaging device, light is incident to the P-type silicon substrate through the flattening layer, the transparent electrode 57, and the insulating layer 55, and after photoelectric conversion is carried out in the N-type region 51 of the photoelectric conversion region 51, the signal charge is stored. The stored signal charges are transferred sequentially by applying pulses to a plurality of transparent electrodes 56.
However, several problems have been encountered in the conventional solid imaging device: the sensitivity is reduced when the transparent film is thick, and the transfer efficiency and the quantity of the transfer charge are reduced when the transparent film is thin. The resolution of the conventional imaging device is also not satisfactory.
The problems are caused by the following factors. In the conventional imaging device, the transparent electrode made of silicon and the like is formed at an uniform thickness, and one of the measures to reduce the wiring resistance around the transparent electrode is to increase the thickness of the transparent film. However, when the thickness of the transparent film is increased, a part of the light incident to the transparent film is diffused, and the transparency of the transparent electrode decreases so that the sensitivity of the imaging device is reduced. If the thin transparent electrode is used in order to increase the sensitivity, the wiring resistance increases, which results in causing a problem of the pulse rounding, and in decreasing the transfer efficiency and the transfer charge quantity. Furthermore, when the structure shown in FIG. 7 is considered, light incident to the periphery of the channel stop region through the transparent electrode is distributed to both of the photoelectric conversion regions of the channel stop region to cause photoelectric conversion, which results in the reduction of the resolution of the imaging device.
It is therefore an object of the present invention to provide a frame transfer-type or a full frame transfer-type solid imaging device and method of manufacturing the same, which is superior in sensitivity or resolution without reducing the transfer efficiency or the transfer charge quantity.
According to the first aspect of the present invention, the solid imaging device, which corresponds to a frame transfer-type or full-frame transfer-type solid imaging device, comprising: a plurality of photoelectric conversion regions and a plurality of channel stop regions for separating each photoelectric conversion region are arranged on a semiconductor substrate; and transparent electroded, which are formed above said plurality of photoelectric conversion regions and said plurality of channel stop regions formed through a first insulating film; wherein said solid imaging device further comprises an antireflection film formed on at least a part of said transparent electrodes located above said photoelectric conversion region.
According to the second aspect, the solid imaging device which corresponds to a frame transfer-type or full-frame transfer-type solid imaging device, comprising: a plurality of photoelectric conversion regions and a plurality of channel stop regions for separating each photoelectric conversion region are arranged on a semiconductor substrate; and transparent electrodes formed through a first insulating film above said plurality of photoelectric conversion regions and said plurality of channel stop regions; wherein, said solid imaging device comprises an antireflection film, having an intermediate refractive index in between two refractive indices of said transparent electrode and a second insulating film formed so as to cover said transparent electrodes, forming at an interface between at least a part of said transparent electrodes located above said photoelectric conversion region and a second insulating film covering said transparent film.
It is preferable that the thickness of a part of said transparent electrode above said photoelectric conversion region be formed so as to be thinner than that of the other area of said transparent electrode. And it is also preferable that a light shielding film is formed on said transparent electrode excluding an area above said photoelectric conversion region.
According to the third aspect, the solid imaging device, which corresponds to a frame transfer-type or full-frame transfer-type solid imaging device, comprising: a plurality of photoelectric conversion regions and a plurality of channel stop regions for separating each photoelectric conversion region are arranged on a semiconductor substrate; and transparent electrodes, which are formed above said plurality of photoelectric conversion regions and said plurality of channel stop regions formed through an insulating film; wherein the solid imaging device further comprises: a light shielding film formed on the surface of said transparent electrode excluding the surface above said photoelectric conversion region.
According to the fourth aspect, the solid imaging device which corresponds to a frame transfer-type or full-frame transfer-type solid imaging device, comprising: a plurality of photoelectric conversion regions and a plurality of channel stop regions for separating each photoelectric conversion region arranged on a semiconductor substrate; and transparent electrodes formed through a first insulating film above said plurality of photoelectric conversion regions and said plurality of channel stop regions; wherein the thickness of said transparent electrode above said photoelectric conversion region is formed so as to be thinner than that of the other area of said transparent electrode.
According to the solid imaging device of the present invention, since an antireflection is disposed on a part of the surface of the transparent electrode located at a position above the photoelectric conversion region, the quantity of light arriving to the photoelectric conversion region through the transparent electrode and the insulating film increases; that is, the transmission increases. As a result, the sensitivity is improved. Furthermore, it is possible to prevent total reflection of light when an antireflection film having an intermediate refractive index between that of the transparent electrode and the second insulating film is used.
In addition to adopting the antireflection film, when a part of the transparent electrode corresponding to the photoelectric conversion region is made thinner than the other part, the light transmission increases, which results in improving the sensitivity. In other words, since the portion of the thickness of the transparent electrode excluding the area corresponding to the photoelectric conversion region is thick in the present invention, the wiring resistance can be reduced when compared to the conventional device, in which the transparent electrode is formed into a uniformly thin film for the sake of transmission. Consequently, it is possible to prevent the read pulse from rounding and to prevent reduction of the transfer efficiency and the transfer charge quantity. In this case, it is necessary to reduce the thickness of the transparent electrode corresponding to the photoelectric conversion region to less than 300 nm, and it is preferable that the thickness is in a range of 150 to 200 nm. If too thick, the low transmission degrades the sensitivity. There is a proportional relationship between the film thickness and the area of the light receiving cell, and if the cell area is constant, the sensitivity increases with decreasing thickness, and if the sensitivity is constant, the cell area may decrease.
When a light shielding film is formed on, for example, the channel stop regions, the light incident to the channel stop regions is shielded. Thus, the resolution of the device is improved because the light incident to a photoelectric conversion region is prevented from being incident to the adjacent photoelectric conversion region.
Practical examples of material used to manufacture the solid imaging device includes silicon for the semiconductor substrate; silicon oxide, silicon nitride, or silicon oxynitride for the (first) insulating film; polycrystalline silicon for the transparent electrode; silicon nitride or silicon oxynitride for the antireflection film; silicon oxide for the (second) insulating film; silicides of high melting point metals such as titanuim silicide or tungsten silicide or other metals for the light shielding film. For example, when polycrystalline silicon is used for the transparent electrode, silicon nitride or silicon oxynitride is used for the antireflection film, and silicon oxide is used for the second insulating film, a combination of the refractive indices of these layers from the bottom is formed as approximately 5, approximately 2, and about 1.45, which satisfy the condition for preventing total reflection.
A method of manufacturing a solid imaging device comprising the steps of: forming a plurality of a photoelectric conversion region by converting a part of a surface of a semiconductor substrate having the first conduction type into a region having a second conduction type, which is an opposite conduction type to said first conduction type, by ion implantation; forming a plurality of channel stop regions having a first conduction type which separates said plurality of photoelectric conversion regions; forming an insulating film on said semiconductor substrate; forming a transparent electrode on said insulating film; and forming an antireflection film having a refractive index smaller than that of the transparent film on a part or the whole of the surface of said transparent electrode. In the manufacturing step of forming said antireflection film, it is preferable to form the antireflection film on an area of said transparent electrode located above said photoelectric conversion region.
The other method of manufacturing a solid imaging device comprising the steps of: forming a plurality of photoelectric conversion regions by converting a part of a surface of a semiconductor substrate having the first conduction type into a region having a second conduction type, which is an opposite conduction type to said first conduction type, by ion implantation; forming a plurality of channel stop regions having a first conduction type which separates said plurality of photoelectric conversion region; forming an insulating film on said semiconductor substrate; forming a transparent electrode on said insulating film; and removing a transparent electrode above said photoelectric conversion region such that the thickness of a part or the whole of said transparent electrode above said photoelectric conversion region is thinner than that of the other area of said transparent electrode. The method further comprises the step of: subsequent to the step of removing a transparent electrode above said photoelectric conversion region such that the thickness of a part or the whole of said transparent electrode above said photoelectric conversion region is made thinner than that of the other area of said transparent electrode; and forming an antireflection film having a refractive index smaller than that of said transparent electrode on a part or the whole of the surface of said transparent electrode.
The still other method of manufacturing a solid imaging device comprising the steps of: forming a plurality of photoelectric conversion regions by converting a part of a surface of a semiconductor substrate having the first conduction type into a region having a second conduction type, which is an opposite conduction type to said first conduction type, by ion implantation; forming a plurality of channel stop regions having a first conduction type which separates said plurality of photoelectric conversion regions; forming an insulating film on said semiconductor substrate; forming a transparent electrode made of polycrystalline silicon on said insulating film; depositing a high melting point metal film on said transparent electrode; forming silicide of said high melting point metal at a portion where the high melting point metal is in contact with said transparent electrode by heat treatment; and removing said high melting point metal film which is not converted into silicide.
The further still other method of manufacturing a solid imaging device comprising the steps of: forming a plurality of a photoelectric conversion regions by converting a part of a surface of a semiconductor substrate having the first conduction type into a region having a second conduction type, which is an opposite conduction type to said first conduction type, by ion implantation; forming a plurality of a channel stop regions having a first conduction type which separates said plurality of photoelectric conversion regions; forming an insulating film on said semiconductor substrate; forming a transparent electrode made of polycrystalline silicon on said insulating film; depositing a high melting point metal film on said transparent electrode; forming silicide of said high melting point metal at a portion where the high melting point metal is in contact with said transparent electrode by heat treatment; and removing said high melting point metal film or the silicide located above said photoelectric conversion region. The method may further comprise the step of: after forming the antireflection film, forming a high melting point metal film on said transparent electrode; forming a silicide of said high melting point metal at a portion where the high melting point metal is in contact with said transparent electrode by heat treatment; and removing said high melting point metal film which is not converted into silicide. The method may further comprise the step of: after forming the antireflection film; forming a high melting point metal film on said transparent electrode; forming a silicide layer of said high melting point metal at a portion where the high melting point metal is in contact with said transparent electrode by heat treatment; and removing said high melting point metal film or the silicide layer located above said photoelectric conversion region.
The still further method of manufacturing a solid imaging device comprises the steps of: forming a plurality of a photoelectric conversion regions by converting a part of a surface of a semiconductor substrate having the first conduction type into a region having a second conduction type, which is an opposite conduction type to said first conduction type, by ion implantation; forming a plurality of a channel stop regions having a first conduction type which separates said plurality of photoelectric conversion regions; forming an insulating film on said semiconductor substrate; forming a transparent electrode made of polycrystalline silicon on said insulating film; forming a high melting point metal film on said transparent electrode; forming a silicide layer of said high melting point metal at a portion where the high melting point metal is in contact with said transparent electrode by heat treatment; removing said high melting point metal film or the silicide layer located above said photoelectric conversion region; and forming an antireflection film having a refractive index smaller than that of the transparent film on the surface of said transparent electrode located above said photoelectric conversion region.
A method of manufacturing a solid imaging device comprising the steps of: forming a plurality of a photoelectric conversion regions by converting a part of a surface of a semiconductor substrate having the first conduction type into a region having a second conduction type, which is an opposite conduction type to said first conduction type, by ion implantation; forming a plurality of a channel stop regions having a first conduction type which separates said plurality of photoelectric conversion regions; forming an insulating film on said semiconductor substrate; forming a transparent electrode made of polycrystalline silicon on said insulating film; depositing a metal film on said transparent electrode; and removing said metal film located above said photoelectric conversion region. The method further comprises the steps of: after forming the antireflection film, depositing a metal film on the surface of said transparent electrode; and removing the metal film located above said photoelectric conversion region. The method further comprises the steps of: after forming the antireflection film, forming an antireflection film having a refractive index smaller than that of the transparent film on the surface of said transparent electrode located above said photoelectric conversion region.
By the use of the manufacturing method of the present invention, it is possible to manufacture the solid imaging device which is superior in sensitivity or the resolution without reducing the transfer efficiency and the transfer charge quantity. When a solid imaging device is manufactured, which comprises features of (1) having a thin part of the transparent film formed above the photoelectric conversion region, (2) having a antireflection film formed on the transparent electrode above the photoelectric conversion region, and (3) having a light shielding film formed on the transparent electrode excluding the area above the photoelectric conversion region, these features can be manufactured in sequence in the order (1), (2) and (3). The order can be changed to the order (1), (3) and (2).
The light shielding film can be manufactured by forming first a high melting point metal film on the transparent electrode made of polycrystalline silicon, and then converting the high melting point metal into the silicide by heat treatment, and by removing the silicide of the high melting point metal on an area of the transparent electrode above the photoelectric conversion region. It is possible, on the other hand, to manufacture the light shielding film made of a metal film by depositing a metal film on the transparent electrode and removing the metal film located on the area above the photoelectric conversion region.